1. Field of the Invention
The invention relates to cleaning equipment, which is used in a cleaning process for manufacturing silicon wafers for semiconductor substrates and in a cleaning process for manufacturing semiconductor devices using semiconductor substrates (hereinafter also referred to as a term "wafer"), and also to a cleaning equipment using a treating solution mixed with an oxidizing gas.
2. Description of the Related Art
Conventional means for removing impurities from wafers include a method to treat with a liquid having an etching action, which can remove impurities present on the surface and in the neighborhood of the surface.
An effective method for removing impurities has a simultaneous action of oxidizing and etching. This cleaning method having the oxidizing and etching abilities is known to include an FPM (DHF/H.sub.2 O.sub.2 : hydrofluoric acid hydrogen peroxide mixture) cleaning method (Japanese Patent Application Laid-Open No. Hei 3-120719) combining DHF (diluted hydrofluoric acid) and H.sub.2 O.sub.2, and a cleaning method using an oxidizing gas instead of H.sub.2 O.sub.2. These methods remove impurities present on the surface-oxidized layer by the following action of DHF. EQU 4HF+SiO.sub.2 .fwdarw.SiF.sub.4 +2H.sub.2 O
Besides, impurities can be removed more effectively by oxidizing Si activated by H.sub.2 O.sub.2 to form a film on the wafer surface.
These cleaning methods have an etching rate determined depending on a temperature of the cleaning liquid. Especially, a DHF/oxidizing gas method, namely a method for cleaning by babbling or dissolving a gas such as O.sub.3 into DHF to directly apply to the wafer, has the dissolving and decomposing of an oxidizing gas affected by the temperature of the cleaning liquid, and an oxidizing rate is also varied as a result. Therefore, there has been proposed a method of cleaning by bubbling the oxidizing gas in order to obtain a sufficient oxidizing effect.
But, according to the above-described cleaning methods, to clean a wafer placed in a carrier, gas is not easy to act on the wafer surface, and especially the gas is difficult to reach the portions where the wafer is contacted with the wafer carrier, and a uniform cleaning effect on the wafer surface and through the wafer plane cannot be obtained with ease.
Therefore, the method of dissolving the oxidizing gas into the cleaning liquid to give an oxidizing effect to the cleaning liquid is effective. But, since the oxidizing gas is easy to decompose, and it is not dissolved with ease.
To solve such problems, the following factors are named.
(a) Henry's law controlling the solubility of gas includes the following:
(1) a concentration of solute: the solubility becomes high when the solute has a high concentration, namely gas has a high concentration;
(2) a kind of solvent: the solubility of gas is variable depending on the presence or not of polarity;
(3) a temperature of solvent: the solubility of gas becomes high when the solvent has a low temperature;
(4) a partial pressure of gas: the solubility becomes high when the gas has a high partial pressure; and
(5) a contact area between solute and solvent: the solubility becomes high when the contact area between the solute and the solvent is large.
(b) Etching rate of silicon by DHF is determined by the following factors:
(1) a concentration of HF: the etching rate becomes high when the concentration of HF is high; and
(2) a temperature of solution: the etching rate becomes high when the solution has a high temperature, and when the treating liquid is flowing to circulate, its flow rate also has some influence.
In the DHF/oxidizing gas method, the etching rate of wafer varies depending on not only the etching rate of HF but also the oxidizing rate of the oxidizing gas. Therefore, it is significant to adjust a temperature of the treating liquid, which is a factor common to the etching rate of HF and the solubility of the oxidizing gas. Besides, to keep the action of the treating solution uniform, it is necessary to adjust the concentration of HF and the dissolving amount of the oxidizing gas.